As a conventional example of a DC power supply device, there has been an LED (Light Emitting Diode) driving device that is disclosed in JP 2011-82204 A (hereinafter, referred to as “Document 1”). The LED driving device in Document 1 performs, with a diode bridge, full-wave rectification for an AC voltage received from an AC power source, and then converts, with a DC-DC converter of a switching-control system, the full-wave-rectified pulsating voltage into a desired DC voltage. In addition, Document 1 discloses that a so-called SEPIC (Single Ended Primary Inductance Converter) is used as the DC-DC converter in the LED driving device.
Incidentally, components other than a switching element, an output capacitor and a diode, of circuit components constituting a SEPIC, form an LC resonance circuit. Specifically, the LC resonance circuit is formed by an input capacitor, a coupling capacitor, a first inductor and a second inductor, as the components. After power from the AC power source is started to be supplied, a resonance current may flow through the LC resonance circuit during a period until start of the switching operation of the switching element. Here the switching element is connected in parallel with a series circuit of the input capacitor and the first inductor, and further in parallel with a series circuit of the coupling capacitor and the second inductor. As a result, when the resonance current flows through the LC resonance circuit as described above, there is a possibility that an excess voltage is applied to the switching element. In particular, the possibility will be remarkable, when power from the AC power source is started to be supplied in a state where a voltage of the AC power source is high.